BOULDER CREEK ASSEMBLAGE, MOUNT BAKER, WASHINGTON: A RECORD OF THE LATEST CONE BUILDING ERUPTIONS

The last major cone building eruptions of Mount Baker ended shortly after late Pleistocene ice melted out of surrounding valleys. Growth of the Baker edifice on the east side of the Pleistocene Black Buttes center resulted in lava-ice interactions. Pyroclastic flows and lahars were shed to the flanks of the volcano. The Boulder Creek Assemblage (BCA; Hyde and Crandell, 1978) filled the ancestral valley of Boulder Creek, on the SE flank of the volcano. The BCA is composed of andesitic lavas, interbedded with cogenetic, coarse boulder-sand diamicts and crudely to well-sorted, stratified, lithic crystal sand. The assemblage lies directly above till deposited during the Vashon stade of the Fraser Glaciation. Subsequent trenching by modern Boulder Creek and its tributaries exposes the assemblage in valley walls.

An 83-meter thick section of the BCA was measured at the type locality on the left bank of Boulder Creek at 490 m elevation, 9 km southeast of the summit crater. At least 12 boulder-sand diamicts, averaging 4 m thick, include andesite clasts to 3 m diameter. Blocks with prismatic jointing indicate that some flows are transitional from block-and-ash flows to granular, water-mobilized lahars. Distal lavas of the BCA are exposed 1 km upstream. Proximal to the cone, lava flows progressively dominate the assemblage. Block-and-ash flows in the type section are likely derived from collapsing fronts of lava flows. Distally, the diamicts and their alluvial runouts debouched from the narrow valley of Boulder Creek into the Baker River valley, forming a fan with a radius of 2.5 kilometers.

Postglacial volcaniclastic assemblages similar in texture to the BCA occur south of Mount Baker, in Sulphur and Pratt Creeks. Fragile, prismatically jointed blocks are conspicuous, but these deposits are not seen to interfinger with lavas.

The BCA is overlain by ash of tephra set SC, (8,800 ¹⁴C yrs BP)from Schriebers Meadow cinder cone, 8 km south of the Baker summit. Post-BCA Holocene deposits from Mount Baker are juvenile and phreatomagmatic tephras, and lahars originating as flank collapses on the southeast half of the edifice.